dietz@cs.rochester.edu (Paul Dietz) (01/24/89)
>> Since putting >> HST into low orbit where the shuttle can reach it reduces the fraction >> of time it can be used by a factor of three... Here is the reference for this: Robert Bless, "Space Science: What's Wrong at NASA", Issues in Science and Technology, Winter 1988-89, pages 67-73. Bless, who is the principle investigator for the high-speed photometer on the HST, says: The HST will orbit about 370 miles above the Earth. This is nearly twice as high as most shuttle flights, but our planet still blocks about half of the sky from the telescope's field of vision. Coupled with other observational constraints, this means the telescope can gather data only about one-third of the time ... By comparison, the smaller International Ultraviolet Explorer satellite [in GEO] can observe the heavens 85 to 90 percent of the time. I'm not sure what the other observational constraints are -- not pointing at the sun or moon, maybe, or perhaps time required to settle after slewing to a new target. Bless mentions that real-time control will be difficult, since TDRSS is a very limited resource and high data rates are available to HST only 15-20% of the time. Placing HST in GEO would have avoided this problem. Bless also explains that returning HST to Earth for repair/relaunch would approach the cost of building/launching a new telescope, and that on-orbit repair missions will cost $250 M each, since the shuttle must be launched empty in order to have enough fuel for two rendevous attempts. Paul F. Dietz dietz@cs.rochester.edu